MTC device triggering is a feature defined by the 3GPP's (Third Generation Partnership Project's) LTE-A (Long Term Evolution-Advanced) (see e.g. NPL 1). The MTC device triggering is sent from SCS (Services Capability Server) or SME (Short Message Entities) to network and terminated at MTC device. MTC device triggering message can be sent in NAS (Non-Access-Stratum) messages, SMS (Short Message Service), or user plane message.
MTC device trigger may not reach MTC device due to security protection check failure at UE. For example, it is described in NPL 2 that some NAS messages (e.g. Identity Request, Authentication Request, Detach Accept, etc.) with no protection can be processed by UE. If a fake MTC device trigger is embedded in such NAS messages, it can cause MTC device battery consumption, and potential mis-behaviour/mis-configuration of the MTC device.
When the secure exchange of NAS messages has not been established, UE discards the NAS messages which do not pass the integrity check (see e.g. NPL 3). When MTC device trigger is carried in such NAS messages and discarded, SCS will not have knowledge about it and may send the same trigger again. This will cause 1) overloading the network, 2) MTC device battery consumption. Another example is when trigger sent over user plane. The current 3GPP security mechanism requires confidential protection on user plane. Similarly problem should be considered that when the user plane message carrying trigger is not properly protected.
There is also considered an issue for SMS based trigger. In LTE where CSFB (CS (Circuit Switched) Fall Back) is in use, and SMS trigger is sent from SCS, without knowledge about if MTC device is IMS (IP (Internet Protocol) Multimedia Subsystem) support, MTC-IWF may forward the message to MME (Mobility Management Entity, assuming it is the serving node), then MME will decide what is the correct route. For example, if the UE is not IMS supported, MME will forward the SMS trigger to MSC (Mobile Switching Centre).